In EP0803224 a suction unit is integrated with a fan unit and a dust chamber, while all components are accommodated in the same housing. In EP0803224 the outlet of the nozzle for removing particles from a surface to be treated communicates with a chamber accommodating a dust container. The chamber is connected to a fan unit that provides under pressure. The nozzle is mounted to the chassis by an arm that is supported by a ball joint so that it can pivot with relation to the housing. During movement of the vacuum cleaner across the floor the nozzle rests by its own weight on the floor and floats on the floor because of the flexible support at the ball joint.
A problem with known suction units is that when the nozzle is completely sealed from the outside atmosphere the under pressure in the nozzle increases while the nozzle gets stuck on the surface to be treated. This occurs especially when cleaning soft floors such as carpets. The problem is already well known for traditional non-autonomous vacuum cleaners. For autonomous vacuum cleaners the consequences generally are worse, since it can lead to a device that gets inoperable, without a user noticing it. The enhanced under pressure results in a normal force that presses the nozzle down to the cleaning surface. It could then occur that the driving system has insufficient power to move the suction unit or vacuum cleaner to overcome the increased downward force. This can result in the device getting immobile. This is in particular true when the suction unit is relatively small, since in that case a drive system normally will only have limited power.